• Refers to the timing of information about cause and effect
• Were info about cause and effect taken at same point in time or was one garnered before the other?
• Demonstrating temporality is difficult in most observational studies

• both are based on exposure and disease info that is collected at same time
• Advantage: efficient for generating and testing hypotheses
• Disadvantage: Leads to challenges regarding interpretation of results
• Cross-sect: has difficulties in distinguishing the exposure from the outcomes of the disease, esp in outcome is biological or physiological
• Case-control: concerns that recall of past exposures differs b/t cases and controls
• none is well suited for uncommon exposure

• a population group, or subset that is followed over a period of time
• group member experience a common exposure associated with a specific setting (occupational or school cohort) or share a non-specific exposure associated with general classification (same birth year or era)

• a tabulation and analysis of morbidity or mortality rates in relationship to the ages of a cohort identified at a particular period of time and followed as they pass thru different ages during part or all of their life span.

• aka follow-up or longitudinal study, prespective, incidence study
• start with group who lack a postive history for the outcome of interest and are at risk for the outcome
• include at least 2 observation points: to determine exposure status and eligibility; and determine # of incident cases
• permit calculation of incidence rates
• can be thought of as going from cause to effect
• involve collection of primary data

• study in which the cohort includes entire population or representaive group of population
• have been used in studies of CHD
• the exposure are unknown until the first observation when exposure info is collected
• there can be two or more levels of exposure
• Ex: Framingham Study, Tecumseh Study, Western collaborative group study

• population cohort study  of CHD initiated in 1949
• used random sample of 6500 ages 30-59
• group members were examined every two years for 30 years
• identified major physical risk factors for CHD

• a total community cohort study
• examined contribution of environmental and constitutional factors to the maintenance of health and origins of illness
• from 1959-60 and enrolled 88% of the community

• cohort of over 5000 heart disease-free CA males for 7 years
• type A personality was at higher risk form CHD
• subsequent study extended over 21 years had contradictory results

• these overcome limitations of pop-cohort, which are not efficient for rare exposures
• certain groups, such as occupational groups, may have higher exposure than general population
• study is made of subjects with a common exposure

True/false

In a cohort study, the comparison group = the exposed group

False

the comparison groups is of the same demographics as the exposed group, but they lack exposure

Several categories of exposure may exist

• purely prospective
• determines exposure at present time and occurrence of disease at a future time
• Advantages: enables investigator to collect data on exposure; most direct and specific test of the study hypothesis
• size of cohort is under greater control
• biological and physiological assays can be performed
• direct measure of the environment can be made (ex: indoor radon levels, EMF radiation)

• make use historical data to determine exposure level at some baseline in the past (prospective must wait for info to accrue)
• significant amt of follow-up may be accrued in short period of time
• amt of exposure data can be extensive and available at minimal cost

• availability of exposure data
• size and cost of cohort used
• data collection and data management
• follow-up issues
• sufficiency of scientic justification

• the investigator, thru direct contact with cohort, must obtain data on subsequent incidence of the outcome (disease, change in reisk factor)
• accomplished thru follow-up mailings, phone calls, or written invitations
• Ex: Minnesota Breast Cancer family study: 2 mailed surveys, postcard reminders and phone calls

Describe passive follow-up

• doesn't require direct contact with cohort
• possible when databases containing the outcomes of interest are collected and maintained by organizations outside the investigative team
• Ex: used in Iowa Women's Health study

true/false

Cohort studeis are the only observational study design that permits examination of multiple outcomes

• it provides a direct measure of association b/t exposure and outcome
• relative risk is the measure of disease in the exposed to the incidence in the nonexposed
• incidence rate = amt with disease/total amt in group w/ and w/o disease ( can be exposed group or unexposed groups)
• relative rate is incidence ratio of exposed group to unexposed

true/false

Attributable risk is also called risk ratio or morbidity/mortality risk

False

Attibutable risk is called risk difference

Relative risk is called risk ratio or morbidity risk

• example of major cohort study
• studied factors with health and mortality
• ages 16-94
• data collected thru mailed questionnaires, telephone/home interviews
• follow ups at years 9, 18, and 29

• Studied coronary heart disease and stroke in men of Japanese decent
• ages 45-65
• data collected thru questionnaires, interviews, clinic examinations

• originally studied oral contraceptive use, expanded to women's health
• married female RNs ages 30-55
• mailed questionnaires
• follow-up every 2 years
• toenails sample at 6 years; blood sample at 13

• type of case control study in which cases and controls are drawn from the population in a cohort study
• population of cohort comprises exposed and nonexposed persons
• provides a degree of control over confounding factors
• reduced cost, but data collected only in subset
• Ex: study of suicide in eletric utility workers

• permit direct determination of risk
• time sequencing of exposure and outcome
• assessment of exposure unbiased by outcome
• can study multiple outcomes
• can study rare exposures

• time consuming and laborous
• costly
• subjects lost to follow-up
• may be impractical for rare diseases